This invention relates to signal demodulation in wireless communication systems. More particularly, this invention relates to apparatus and methods for improving the assignment of parallel demodulators to multipaths of wireless signals.
Wireless communication systems typically include pluralities of stationary transceivers (transmitter/receivers), known as base stations, and pluralities of mobile transceivers, known as mobile stations. Such systems control user access to typically shared communication channel capacity by using multiple access communication protocols. One such protocol is CDMA (code division multiple access).
In CDMA wireless systems, each mobile station communicating with a base station is assigned a unique code. This unique code is used to encode data transmitted from the base station to the mobile station assigned that code. Encoded data is transmitted via a modulated signal, which is a signal that has one or more of its wave characteristics (e.g., frequency, amplitude, or phase) modified to represent the encoded data. Upon receipt of a modulated signal, the encoded data is retrieved from the signal by a process known as signal demodulation. The encoded data is then decoded to extract the transmitted user information.
CDMA is used in both cellular and PCS (personal communications service) mobile communication networks. These types of networks divide geographic areas into cells. Each cell has a base station that communicates with mobile stations in that cell. A PCS network typically has smaller cells and operates at lower power and higher frequencies than common cellular networks. CDMA wireless systems typically adhere to either EIA/TIA IS-95, a digital CDMA standard for U.S. cellular radio systems published by the Electronic Industries Association/Telecommunications Industry Association, or ANSI J-STD-008, a similar standard for PCS networks published by the American National Standards Institute.
As mobile stations move from cell to cell, they are xe2x80x9chanded-offxe2x80x9d from one base station to another. In CDMA wireless systems, such hand-offs are xe2x80x9csoft.xe2x80x9d A soft handoff is one in which a mobile station first establishes communication with one or more base stations from cells that the mobile station may be entering before terminating communication with the base station from the cell that the mobile station is exiting. A mobile station in soft handoff can therefore be communicating with several base stations simultaneously. Such base stations are known as xe2x80x9cactivexe2x80x9d base stations and are in the xe2x80x9cactive setxe2x80x9d of that mobile station.
Each base station in soft handoff with a mobile station transmits the same information substantially simultaneously to that mobile station. Transmissions from the mobile station are separately received and demodulated by each those base stations. The demodulated transmissions that meet quality criteria are then combined and passed on to a mobile switching center. A mobile switching center switches transmitted data between wireless and wire-based networks (e.g., a public switched telephone network).
CDMA wireless systems are characterized by multipath signal propagation. Multipath signal propagation is a phenomenon that occurs when a base station transmits a signal to a mobile station and the mobile station receives multiple copies of that signal.
These multiple copies are typically created by signal reflection, and travel to the mobile station via different paths through the air. These copies, commonly referred to as multipath components, or simply multipaths, are separated from each other by small time intervals. These time intervals measured from a common reference are commonly known as xe2x80x9cphasesxe2x80x9d of the transmitted signal. Thus a signal spread over several multipaths can be received by a mobile station at several phases, each phase corresponding to a multipath.
A mobile station can receive multipaths of a signal from either a single base station or several base stations. For example, assume a mobile station is in soft handoff with three base stations. If a signal transmitted from each base station results in two multipaths from the first base station, three multipaths from the second base station, and two multipaths from the third base station, the mobile station can receive the transmitted signal via seven multipaths. Multipaths associated with a particular base station can be distinguished from multipaths of other base stations by the spreading sequence (i.e., code) used to modulate the signals transmitted by that base station.
Signals transmitted by base stations in CDMA wireless systems based on the IS-95 or ANSI J-STD-008 standard typically include several communication channels, such as a pilot channel, other overhead channels (e.g., synch and paging channels), and traffic channels. Traffic channels carry information transmitted by users. Each base station in the active set of a mobile station assigns a particular traffic channel to a mobile station. That traffic channel is modulated by a unique xe2x80x9cWalshxe2x80x9d code. The pilot channel, on the other hand, is modulated by a Walsh code known by all mobile stations. The pilot channel provides a phase reference that enables mobile stations to perform coherent demodulation of signals transmitted to them over their assigned traffic channel.
Mobile stations in CDMA wireless systems typically include CDMA terminals. A CDMA terminal receives, demodulates, combines, and decodes traffic channel signals received via different multipaths of a wireless signal from one or more active base stations. Note that such traffic channel signals can be combined because the same user information is carried by each multipath.
A CDMA terminal typically includes a xe2x80x9crakexe2x80x9d receiver, which has a plurality of parallel demodulators (known as xe2x80x9cfingersxe2x80x9d). Each demodulator can be independently tuned (i.e., assigned) to a particular phase to demodulate traffic channel signals received at that phase. The CDMA terminal combines the demodulated signals from the assigned demodulators, and then decodes the combined signal to extract the transmitted information, which may be, for example, a voice communication. By combining separately demodulated signals in this manner, each carrying the same user information, the signal-to-noise ratio is enhanced, usually resulting in high quality wireless communication.
However, the exact phases at which multipaths appear at mobile stations are generally not known by the mobile stations. The presence of a multipath at a particular phase can be determined by measuring the power received at that phase. Furthermore, to improve the signal-to-noise ratio, and thus improve communication quality, demodulators should preferably be assigned to phases at which the strongest multipaths appear. Demodulators assigned to weak or non-existent multipaths contribute primarily only noise to the process of combining demodulated traffic channel signals. The strength of a multipath is indicated by the power level of the pilot channel signal contained within that multipath. CDMA terminals therefore continually measure power at various phases of received pilot signals to first determine whether multipaths are present and then to preferably identify the strongest multipaths.
To conduct initial power measurements, rake receivers typically have a search demodulator, commonly known as a xe2x80x9csearch finger,xe2x80x9d in addition to the parallel demodulators used for traffic channel demodulation. The search demodulator typically follows a schedule to measure pilot channel power at various phases within search windows of several base stations. A search window is a period of time encompassing the probable phases at which multipaths from a particular base station may appear. A relatively high value of measured power at a particular phase within the search window of a base station generally indicates the presence of a multipath from that base station at that phase. Demodulator assignment logic within CDMA terminals analyzes the various power measurements to determine the pilot signal phases at which to assign demodulators.
However, these power measurements are statistical and are typically subject to noise and interference from outside sources (e.g., transmissions from other base stations). This noise and interference can result in power measurements that seem to indicate the presence of a multipath at a particular phase when in fact none exists at that phase. This is known as a false alarm.
Moreover, as mobile stations move, the strength of received multipaths can fluctuate because of a phenomenon known as fading, which is caused by signal reflection, refraction, or absorption. Also, as a mobile station moves away from a base station, signals from that base station weaken (received power decreases). Conversely, as a mobile station moves toward a base station, signals from that base station intensify (received power increases). Multipath signal strength can also fluctuate when, for example, a mobile receiver moves through a tunnel or hilly terrain. Therefore, although a single momentary power measurement may indicate the presence of a reasonably strong multipath, that multipath may be weakening and would thus be a poor choice for signal demodulation.
Accordingly, the ability of a CDMA terminal to provide high quality wireless communication is dependent, in large part, on the ability of its demodulator assignment logic to assign demodulators to the strongest received multipaths. However, a disadvantage of known demodulator assignment logic is an inadequate ability to distinguish strong multipaths from false alarms or quickly weakening multipaths. A quickly weakening multipath is one in which initially measured power at least equals a power threshold, but which drops below the threshold within a short period of time. This inadequate ability can result in the erroneous assignment of demodulators to weak or non-existent multipaths. Such erroneous assignments adversely affect communication quality, because, as noted above, those erroneously assigned demodulators contribute primarily only noise to the signal combination process. Furthermore, if a demodulator is assigned to a false alarm or a quickly weakening multipath, a good possibility exists that a stronger multipath is being received by the CDMA terminalxe2x80x94but which is not being used for signal demodulation. This disadvantage of known demodulator assignment logic is amplified in environments where a large number of pilot signals are rapidly fluctuating.
In view of the foregoing, it would be desirable to provide apparatus and methods for improving the assignment of parallel demodulators to multipaths by reducing the likelihood of assigning a demodulator to a false alarm or a quickly weakening multipath.
It would also be desirable to provide apparatus and methods for improving the assignment of parallel demodulators to multipaths by reducing the likelihood of processing demodulated data from a demodulator assigned to a false alarm or a quickly weakening multipath.
It is an object of this invention to provide apparatus and methods for improving the assignment of parallel demodulators to multipaths by reducing the likelihood of assigning a demodulator to a false alarm or a quickly weakening multipath.
It is also an object of this invention to provide apparatus and methods for improving the assignment of parallel demodulators to multipaths by reducing the likelihood of processing demodulated data from a demodulator assigned to a false alarm or a quickly weakening multipath.
In accordance with this invention, apparatus is provided for assigning parallel demodulators to multipaths of a wireless signal. The apparatus includes at least one parallel demodulator, control circuitry coupled to the demodulator, and memory coupled to the control circuitry. The memory includes a multipath signal database and demodulator assignment logic. The assignment logic creates an unconfirmed entry in the database corresponding to a phase of an active pilot signal (i.e., the pilot signal from an active base station) when power measured at that phase at least equals a first selectable power threshold. The assignment logic deletes an unconfirmed entry from the database when at least one additional power measurement at a phase of an active pilot signal corresponding to the unconfirmed entry is below the first power threshold. The assignment logic confirms an unconfirmed entry when at least one additional power measurement at a phase of an active pilot signal corresponding to the unconfirmed entry at least equals the first power threshold. Only after an entry is confirmed, does the assignment logic assign the at least one demodulator to the corresponding phase of the active pilot signal of that entry.
Apparatus is also provided in which demodulator assignment logic permits processing of demodulated data from a demodulator assigned to a phase of an active pilot signal when power measured at the phase after the demodulator is assigned to that phase at least equals another selectable power threshold. If the measured power is below that threshold, the assignment logic releases the demodulator from that phase.
Furthermore, apparatus is provided that includes selectable first and second parameters for varying the degree to which a multipath""s presence and quality are confirmed before assigning a demodulator to the corresponding phase. Apparatus is also provided that includes a live-set database for creating and updating entries corresponding to phases of active pilot signals that have been assigned parallel demodulators.